Earlier paper packaging bags have been replaced with plastic bags in many fields of heavy duty flexible packaging application (e.g., packing of fertilisers, polymer pellets, animal feed etc). One of the advantages of plastic is that plastic (e.g. film or woven fabric) bags can be closed with welding after filling. That is one of the factors that have lead to their use in modern automated Form-Fill-Seal (i.e., FFS) packaging. A drawback of plastic bags, however, is that they are more slippery than traditional kraft paper bags which causes problems at their stacking involving laying the bags (especially if filled with such easily-flowable materials as e.g. dry salt or dry classified quartz sand) onto each other on a pallet. There have been several approaches for making plastic bags antislip.
Plastic bag outer surfaces can be tackified or provided with spots or strips of a coating, or print, of such a polymer or ink as has a substance of a high coefficient of friction, e.g. with spots or strips of such hot melt or water-based glue coating. It can decrease the slip between the bags. The substance must be tacky and/or elastomeric to provide an antislip effect. In such products the high-coefficient-of-friction coating is essentially two-dimensional and as flat and thin as possible, because making it to be thicker would not improve its antislip character but would increase it costs. In our understanding it means that in these cases even if the surface is provided with a certain roughness or texture on the micro-scale, it is the substance (e.g. a hot melt polymer) of the textured surface that provides an antislip effect, rather than the geometry of the texture, and a height of the coat texture is typically much lower than about 50 micrometres, most often it is up to about 10 micrometres. One example is in DE19938828A1.
Such antislip effect can be spoiled under dusty or wet circumstances and also the generally soft antislip substance is usually not abrasion-resistant enough.
In US20150036952A1 as anti-skid material, heated, liquefied Vistamaxx™ 6202 (an easily extrudable olefinic elastomer grade from ExxonMobil Chemical) is applied from an extrusion head to a woven polypropylene packaging bag, and immediately flattened with rollers, in plural spaced apart strips with the aim of increasing the coefficient of friction of the surface. We think that here also, a high coefficient of friction of the elastomer necessarily comes together with a low wear- or abrasion resistance of the same. A final layer thickness of the strips is not stated in the text, and the figures are not to scale. In our understanding, however, if the layer thickness of the strips were great enough to provide an antislip effect based on a mechanical interlock between mating strips of mating bags (independently from the substance of the strips being elastomeric) then the long continuous strips would severely compromise the bag's flexibility, and in addition the antislip effect based on the mechanical interlock would lack any isotropy, further it would be very expensive.
Roughened plastic flexible packaging materials are known to have been used in individual-bag as well as FFS packaging also in industries packing dusty products. In such materials the antislip effect is based on an essentially three-dimensional texture of the rough surface while simultaneously a flexibility of the packaging material should be maintained. Namely, protruding parts of the antislip surface interlock with a mating surface. Necessarily, typically the higher and sharper the protrusions are, the better the antislip interlock is. Typically a protrusion height of at least about 50 micrometres is useful, and a presence of an undercut in the protrusions (as seen in a side view of the protrusion) makes the antislip effect much better.
In a first group of roughened antislip plastic flexible packaging material solutions the roughening texture is formed from the material of the plastic wall of the packaging material itself. In DE 3437414 A1 embossing pins are used to raise individual points of the film, in U.S. Pat. No. 3,283,992 linear ribs are raised from the original surface, in U.S. Pat. No. 6,132,780 an annular ring surrounds a perforated aperture in the film. Drawbacks thereof include that a desirable sharp, undercut character of the roughening protrusions can not be provided, further, the substance of the antislip protrusions is inherently identical with that of the wall, and the wall can be weakened. Further, with antislip protrusions of rib-like, elongated shape (as seen in a top view thereof) generally a desirable isotropy (i.e., providing uniform antislip engagement in all shearing directions) of the antislip surfaces can not be provided and also a flexibility of the packaging material is compromised.
In a second group of roughened antislip plastic flexible packaging material solutions the roughening texture is formed from a material other than the material of the plastic wall or of the essential strength-giving layer or -component of the packaging material. In U.S. Pat. No. 4,407,879 there is blown a film from a polyethylene blend whose two components are compatible (i.e., in our understanding, have identical melt mass flow rates) to be extruded together well, and have dissimilar (higher and lower) softening temperature points, and the film is reheated to between the two softening temperature points in order to re-melt the blend-component of the lower softening temperature point and thereby roughen the film's surface. The document focuses on the teaching that a polymer is easier and faster to melt if it has a lower softening or melting temperature. The document does not mention any melt mass flow rate values of the polymers but implicitly suggests a use of polymers of a great melt strength (i.e., a very low fractional melt mass flow rate) in order of keeping the partially re-melted film from breaking. The drawbacks of the method are its complexity and that the film's strength appears to be difficult to maintain with regard to the heat treatment. In U.S. Pat. No. 4,488,918, making a coextruded blown packaging film with a roughened antislip outer surface includes using in an outer coextrusion layer a high density polyethylene grade having an expressly low fractional melt mass flow rate preferably as low as 0.14 or preferably even lower, which is taught to be fundamental to its roughening effect. (Fractional melt mass flow rate is a name used for melt mass flow rate values lower than 1.0). The method is complicated, provides imperfect isotropy of the antislip effect, and for an acceptable antislip effect a thick ruptured coextrusion layer is necessary which compromises a flexibility of the material and is costly. Anotherway of making packaging films with a roughened antislip outer surface is mixing, in the extruder, relatively solid particles into the film's melted substance which finally results in solid protrusions in the outer surface of the film. The particles must survive the extrusion therefore their substance is either non-thermoplastic, such as mineral (being possibly harmful to the extruder and to the film's recyclability) or such thermoplastic as has an extremely high melt viscosity, i.e., an extremely low melt mass flow rate (see e.g. U.S. Pat. No. 7,314,662). Another method, described in WO 8901446A1, of roughening a polyethylene plastic bag wall includes printing a hot melt adhesive, forming “dots”, or more exactly, truncated cones and hemisphere-like protrusions on the film. Its apparatus is a gravure printing roll with cups of 120 micrometres depth and 1 mm diameter. The printed “dots” must implicitly have a diameter of 1 mm, corresponding to the diameter of the cups of the gravure roll, and their height is described to be 50-150 micrometres, thus they are definitely low-profile, flat protrusions (meaning that the document's figures must not be in scale). Hot melt printing, in general, inherently needs melt polymers of a very low melt-viscosity, i.e., of a very high melt mass flow rate in order of allowing a handling of the melt in the apparatus (i.e., filling and emptying the gravure cups, pumping and filtering the melt etc.). (We note that a material having a low melt-viscosity, or high melt mass flow rate, does not necessarily have a low melting temperature, because the two parameters are essentially independent from each other.) Just because of the extremely low viscosity of the liquid melt, hot melt printing processes are inherently unsuitable for forming any high, or sharply protruding, undercut protrusions.
There is a third group of roughened antislip plastic flexible packaging material solutions. These are based on the concept of dispersing and fixing thermoplastic roughening particles, of a size suitable for the slip-decreasing purpose, on the surface of a plastic film or fabric. (Any such use of non-thermoplastic particles would be undesired due to a spoiling of a recyclability of the product). Namely, PCT publication WO 98/34775 and corresponding U.S. Pat. No. 6,444,080 (originating from the current inventors) describe an antislip packaging film comprising a polyethylene film wall and antislip protrusions projecting from its surface, the protrusions constituted by thermoplastic polymer particles fixed to the wall's surface. According to their teaching, the material of the particles can be the same as that of the film wall (note: e.g. polyethylene and polyethylene) or it can also be another plastic material capable of welding with the film wall (note: e.g. polyethylene and polypropylene). Further, the particles can also be adhered to the surface. It means that the substance of the particles can be selected independently from the substance of the film wall. According to the teaching of the documents, the particles must have a good abrasion resistance and also a suitable size (preferably a narrow size fraction). Hungarian publication HU200202948A2 (originating from the current inventors) describes similar roughened packaging films and teaches to use reactor powder as the particles. Granted Hungarian patent HU222597B1, corresponding to WO99/36263 (originating from the current inventors), mentions a polypropylene fabric with an antislip plastic layer comprising protrusions welded into and sticking out of the surface, consisting of the substance of the antislip layer and/or other particles capable of welding with the material of the antislip layer, wherein the antislip layer is fixed to the polypropylene fabric by coating in a moulded condition. In a process example of U.S. Pat. No. 6,444,080 in a film blowing apparatus a polyethylene tubular film is drawn up to form a balloon. For a roughening, polyethylene particles are brought to the film surface before a blowing of the cooling air onto the film. Patent HU220997B1 (originating from the current inventors) describes, in detail, an apparatus for such roughening of a film during the manufacturing of the film wall. In HU220997B1, Page 4, Column 1, lines 26-36 cite (see its FIG. 2.): “Since we displaced, by lifting it up, the cooling ring 20 from a plane of the die gap 2, the film tube 23 remains of essentially the same diameter from having left the die gap 2 up to the cooling ring 20. This section is usually called a neck of the bubble. A height of the mentioned neck can be modified through the haul-off speed and/or a selection of a place of the cooling and/or an intensity of the cooling. The film tube 23 is fully plastic-state in its section between the cooling ring 20 and the die gap 2, i.e., in its neck. This is the section where we get the polymer particles 24 onto the plastic-state film tube 23.”. Further its Page 4, Column 2, lines 27-33 cite (see its FIG. 2.): “An extent of a melting of the polymer particles 24, and thus also of their welding into the surface of the plastic-state tube 23, can be influenced with displacing the cooling ring 20 in a direction of the moving of the film tube 23. Thus, as a certain period of time free of cooling is provided for the polymer particle 24, it welds into the surface of the film tube 23, and thereby a desired roughening can be achieved.” Its FIG. 2., in accordance with its description, shows the melted neck of a plastic bubble emerging from a film-blowing die, and shows the particles being blown, at a first altitude above the die, onto a section of the neck where the neck essentially has a cylindrical shape, and shows a cooling ring at a second altitude significantly high above the place of the blowing of the particles. Particles bouncing from the bubble fall into a suction unit (signed 34 in FIG. 2. of HU220997B1) and are vacuumed away, collected and re-used for the same purpose. From the patent the skilled person will understand that providing a long cylindrical stalk or neck of the bubble is necessary, namely for providing for the dispersed particles a period of time free of cooling.
Further, the ISO 1133-1 standard describes the standard method of determination of the melt mass-flow rate (MFR) of thermoplastics using an extrusion plastometer. The ISO 1133-1 standard prescribes that if melt flow properties in regard of a plastic film are to be measured then some small pieces of the film must be cut, by default, into strips and compacted before measuring.
Our considerations about the above-mentioned second and third groups of known solutions are as follows. Packaging films taught in the third solution group can have the advantages over those of the second solution group that the (mono- or coextruded) layer(s) of the film base wall can be substantially continuous, uninterrupted and having a substantially uniform layer-thickness and, therefore, a good load bearing even adjacently the protrusions fixed thereupon. Further, the protrusions can, in their top view, have a granule-like (rather than e.g. fibre-like or rib-like or ridge-and-valley-like) shape, and they can sharply stick out of the surface of the film base wall, preferably even having undercuts. Solid roughening protrusions taught in the third group of solutions can be realized with good abrasion resistance with a higher ratio of protrusion height to protrusion width than in other solutions (e.g. from the second solution group). All that provides an excellent antislip effect, in fact better than in the second-group solutions. Further, in the third group of solutions the size of the protrusions and also their closeness in the surface can be selected relatively freely. Further, within the field of roughened flexible packaging materials the fact that the antislip protrusions can be of a substance selected relatively freely, and independently from that of the film wall, is a unique feature of the third group of solutions, therefore at selecting substance parameters of the antislip protrusions/roughening particles the skilled person would primarily lean on the teachings of the third-solution-group documents. Nevertheless, any suggestions available from the second group of solutions would appear to suggest using in the outmost, roughening layer of the film a substance of a very low fractional melt mass flow rate.
We note that at comparing viscosities of polymer-melts based on their melt mass flow rate values it is important to keep in mind that the values should be compared with a logarithmic approach. Namely, if we measure and compare, in accordance with the ISO 1133-1 standard, how many grams, of the given melts, flow through the test orifice within ten minutes we will see, for example that a polymer of a mass melt flow rate of 0.20 will produce a flow mass twice as great as a polymer of a mass melt flow rate of 0.10, despite the fact that there is only an absolute numerical difference of 0.10 between the two mass melt flow rate values. It means that at comparing polymers, it is essentially a ratio, of their melt mass flow rate values, rather than a difference thereof, that counts. In other words, there is a greater difference, in this respect, between melt behaviours of two polymers of respective melt mass flow rate values of 0.20 and 0.10 than between those of 200 and 150. As concerning selecting a melt mass flow rate value of the antislip protrusions/roughening particles, there is not any explicit teaching available from the background documents mentioned in the third solution group. A melt mass flow rate of the film wall material is not mentioned in the documents of the third solution group, but the inevitable, elevated cooling ring arrangement and also the shape of the bubble of FIG. 2. of HU220997B1 suggest to the skilled person a film wall material necessarily of a very low fractional melt mass flow rate. We were able to reproduce the film blowing configuration of HU220997B1, with the bubble shape described and illustrated in it, with a polyethylene film of a melt mass flow rate of 0.2 g/10 min. determined at 190° C. under a load of 2.16 kg in accordance with ISO 1133-1. The demand for a good abrasion resistance of the particles (taught in U.S. Pat. No. 6,444,080) leads the skilled person toward a particle-substance of a low fractional melt mass flow rate since a very low melt mass flow rate is known to mean greater product abrasion-resistance and strength (about this see the last paragraph of “Melt Index Mysteries Unmasked (as published in Film Lines (Canadian Plastics Industry Association, Winter 2003))” at internet link “http://www.griffex.com/Griff-meltindex.pdf”). Certain documents in the third solution group teach and suggest the use of reactor powder. That factor, again, leads the skilled person towards very low fractional-melt-flow-rate powders rather than towards higher-melt-flow-rate powders, just like the following factor does: in HU220997B1 the particles bouncing from the hot bubble are re-collected by vacuum and re-used again for the same purpose, such re-used particles making up a very considerable portion of the total amount of powder (see HU220997B1 FIG. 2 sign 24, low left side corner). According to our practical experience the bouncing particles are inevitably hot and therefore prone to sticking together during their re-collection unless they have an expressly low fractional melt mass flow rate, as would also be clear to a skilled person based on trial and error. We managed to perform the operation of the configuration taught in HU220997B1 FIG. 2 with a high density polyethylene reactor powder of a melt mass flow rate of 0.25 g/10 min. determined at 190° C. under a load of 2.16 kg in accordance with ISO 1133-1. Summing it up, there is not any teaching or suggestion prompting (explicitly or implicitly) the skilled person to use in a roughened packaging film, belonging to the third solution group, thermoplastic antislip protrusions of a substance of a melt mass flow rate as high as, or higher than, 0.6 g/10 min while there are definite suggestions on the contrary.
There is still a need to expressly select a melt mass flow rate in the substance of the antislip protrusions used in the solutions of the third solution group.
Further, it is a part of our recognition that we found new problems to solve. These problems, to be solved, relate to antislip packaging bags and to methods and apparatuses for providing plastic packaging bags, as follows.